Casein Kinase from the Golgi Apparatus of Lactating Mammary Gland

Abstract

A casein kinase that catalyzes the phosphorylation of dephosphorylated α_s1-casein by ATP has been found in the Golgi apparatus of lactating rat mammary gland. Dephosphorylated β- and κ-caseins are also phosphorylated by this enzyme, while other milk proteins (β-lactoglobulin, α-lactalbumin, native α_s1-, β-, and κ-casein, and proteins of the fat globule membrane) are phosphorylated to a limited extent. Histones, phosvitin, and lysozyme are not appreciably phosphorylated. The optimum pH for phosphate incorporation into dephosphorylated casein is 7.6. The K_m for dephosphorylated α_s1-casein is 12 µm (.27 mg per ml), whereas the K_m for ATP is 80 µm. The casein kinase requires a divalent cation for maximum activity; both Ca²⁺ and Mg²⁺ can satisfy this requirement. Adenosine 3' : 5'-monophosphate (cyclic AMP) has no effect on casein kinase activity. The protein inhibitor from rabbit skeletal muscle, which inhibits adenosine 3' : 5'-monophosphate-dependent protein kinases, is also without effect. These findings suggest that phosphorylation of casein, a food protein, is quite different from the phosphorylation of cellular enzymes, which requires cyclic AMP, is inhibited by Ca²⁺ and is involved in control mechanisms.